I like to pride myself on being highly self-aware, and if I actually were, that would be a hell of a thing to take pride in. But, I'm not. The only way I know of to gauge how I'm feeling is open an extra tab on my laptop while I'm working and search for songs. I don't know what kind of mood I'm in until I hear what I've selected for myself.
Something Corporate or New Found Glory: nostalgic for my teen years
My Chemical Romance or Green Day: either melancholy or exuberant--they have a wide range
The Beatles or Queen: nostalgic for my childhood
Beyonce/Lady Gaga/P!nk/Katy Perry: amped up and vaguely pissed at the male gender. Tends to occur once a month.
Broadway tunes: highly energetic. Often involves dancing. It's rarely, if ever, pretty
Avenged Sevenfold: angry
Celtic Woman: Irish (yes, Irish is a mood)
As special as I like to believe I am, I wonder if I'm the only one who gauges their own mood by the choices they subconsciously make. Feel free to comment if you do the same, and with what.